The invention relates to a microlithographic illuminating system having an optical axis and optical elements arranged symmetrically with respect thereto as well as an image field plane. The invention also relates to a microlithographic projection exposure arrangement.
Illuminating systems and projection exposure arrangements of the above kind are described, for example, in U.S. Pat. No. 6,243,206.
Such an illumination system is also known from U.S. Pat. No. 6,466,303. The illuminating system and projection objective have a common optical axis indicated by reference character A in FIG. 1 of this publication.
Objectives with mirrors cannot simultaneously have unvignetted field and image planes centered about the optical axis (except when using a beam splitter). For this reason, for example, in U.S. Pat. No. 6,466,303, the object and image field is a rectangle lying outside of the optical axis. The illuminating field, which is generated by the axially-symmetrical illuminating system, includes this off-axis rectangle as shown in FIG. 2 of U.S. Pat. No. 6,466,303.
This circularly-shaped field is significantly larger than a minimum circularly-shaped field which centrally includes the rectangle. The same applies to object and image fields formed as annular segments. These object and image fields are also used in microlithography.
The sizes of the lenses and mirrors of an optical system increase with the field. For this reason, the image errors and the complexity of correction also increase.
In high-power VUV optics, such as for the microlithography at 193 nm or 157 nm excimer lasers, the availability of the optical materials (quartz glass and fluoride single crystals) and the high material cost together with increasing lens diameter become more problematic.
In view of the above, it is an object of the invention to provide an illuminating system which can supply projection objectives having an off-axis field with minimum lens diameters.
If one simply takes an axial-symmetrical illuminating system the optical axis of which centers to the off-axis field of the projection objective but is displaced parallelly to the optical axis thereof, then the following problem occurs.
The imaging errors of optical systems comprising axial-symmetric elements are axially symmetric. If the optical axes of an illuminating system and a projection objective are, however, displaced parallelly to each other in favor of the small illuminating system, then the errors of the two are not of the same symmetry and can therefore not be mutually compensated. For microlithographic projection exposure arrangements, the telecentric error is especially critical.
According to a feature of the invention, the illuminating system and the projection objective are adapted to each other in the symmetry of their respective image error distribution in the reticle plane notwithstanding the axis offset.
A mutual tilting is, in principle, not sufficient.
In the objective, an element, which is not rotationally symmetric to the optical axis, would perforce have influence on the structure transfer whose quality is, however, primary.
The microlithographic illuminating system of the invention defines an optical axis and includes: a light source for supplying and transmitting a light beam along the optical axis; a plurality of optical elements arranged along the optical axis downstream of the light source for conducting the light beam on a beam path along the optical axis and for defining a field; and, one of the optical elements being arranged close to the field for intercepting all of the light beam and being configured so as to be asymmetrical with respect to the optical axis.
The object of the invention achieved with an optical element which is mounted close to the field in the illuminating system as described above and which element is configured asymmetrical to the optical axis. This element is different from an off-axis raster element (as it is used, for example, in honeycomb condensers of illuminating systems) in that, in the element of the invention, the total beam is detected by a single continuously differentiable cross-sectional area. Such elements are especially lenses, prisms, plates and mirrors all with aspheric surfaces of a rotational symmetrical nature as well as free-formed surfaces. Equivalents are like-functioning Fresnel lenses, binary or stepped optical elements and diffractive elements, also in combination.
According to one embodiment of the invention, the asymmetrical optical element is arranged next to the image plane of the illuminating system or to a field plane equivalent thereto.
Based on function, the correction element (asymmetrically configured optical element) would lie optimally precisely in the reticle plane. The optical elements of the illuminating system, however, have to exhibit a practical working distance to the reticle. General requirements of the optical design are also to be considered which can lead to an arrangement of the asymmetrical refractive optical element at deviating locations but not close to the aperture.
Preferred embodiments of the invention provide for a decentered spherical or rotational symmetrical aspheric lens, for example, a toric lens or a wedge plate (prism) as well as a free-form corrective surface (general asphere) in combination with the above-mentioned forms or independent therefrom.
Known centering lenses are decentered in the adjustment of optical systems in the micrometer range in order to bring the assembled system with manufacturing tolerances close to the constructed ideal axially-symmetrical desired system. In contrast to these known centering lenses, comparatively large decenterings of over 1 mm are preferred in accordance with the invention.
According to another feature of the invention, the asymmetrical or decentered element is mounted close to the field in a REMA objective, preferably as the first or last lens of this REMA objective with the REMA objective defining the end of the illuminating system.
According to another feature of the invention, the illuminating system is telecentric with a defined deviation and therefore the illuminating system is adapted to a corresponding projection objective.
The combination of such an illuminating system and the additional conventional systems results in a projection exposure arrangement according to an embodiment of the invention. The number of the system components is not fixed and not limited to the optical core assemblies.
In addition to catoptric objectives, preferably catadioptric projection objectives can be used in accordance with a feature of the invention. Preferred embodiments of the catadioptric objectives are those having precisely one concave mirror and at least two folding mirrors, such as described in U.S. Pat. No. 6,466,303, or embodiments without folding mirrors with a through symmetrical axis of all optical elements. Thus, the projection objective has a symmetrical axis to which the optically effective surfaces of all optical elements are rotationally symmetrical with respect to surface curvature except for production deviations in the individual case.
Suitable objectives, namely catoptric or catadioptric objectives derived therefrom, having a plurality of curved mirrors are known, for example, from U.S. Pat. No. 5,815,310 or U.S. Pat. No. 4,701,035.
According to another feature of the invention, the optical axis of the illuminating system and the optical axis of the projection objective exhibit a parallel offset. According to still another feature of the invention, the image and object circles, which are coincident on the reticle, are of different sizes and the image field of the illuminating system is smaller. The image and object circles include the object field which is to be transmitted. In this way, all advantages are provided in the manufacture of illuminating systems.
According to another feature of the invention, the projection objective is telecentric at the object side with a maximum deviation of the chief ray angles of 0.1 to 50 mrad from the parallel to the optical axis via the object field utilized. Thus, this feature gives the object side telecentry of the projection objective for the appropriate telecentry of the illuminating system. This system is telecentric in the image field plane with a maximum deviation of the chief ray angle of 0.1 to 50 mrad and it is preferable to use this for the invention.